This invention relates to digital oscilloscopes in general, and more specifically to apparatus and method for acquiring data using random repetitive sampling.
A digital oscilloscope samples an analog waveform at discrete points in time, converts the analog values to digital values and stores these values and the corresponding sample times in a memory. The oscilloscope display is a plot of the digital samples as shown, for example, in FIG. 1. The display accuracy depends on the density of the sample points relative to the period and rise time of the sampled waveform. In single shot sampling oscilloscopes, such as the Hewlett Packard Company Model 54200A/D, the density is limited by the maximum sample rate of the oscilloscope. Repetitive sampling oscilloscopes, such as the Hewlett Packard Company Model 54100A/D, use random repetitive sampling to increase the display density of a sampled repetitive waveform beyond the maximum sample rate. In random repetitive sampling, samples taken during several acquisition periods are combined relative to common trigger points to form one displayed waveform. The display density is increased since the acquisition periods overlap relative to identically located trigger points. FIGS. 2A-C show a display after one, two, and multiple acquiaitions, respectively.
If, in random repetitive sampling, the waveform period is shorter than the oscilloscope sample clock period, and the search for a common reference point for the acquisition period begins at a time synchronized to the sample clock, then the displayed information will contain gaps as shown in FIG. 3. Gaps occur because a trigger event will always be located within one waveform period for repetitive waveforms. Phase shifting the sample clock was a prior art solution used in the Hewlett Packard 54100A/D. This solution was bulky, expensive, slow, and required special software. In addition, intermittent display gaps appeared.